At the last meeting I talked about a few tips you should keep in mind when dealing with LiPo batteries. In this article I want to point out one of the most important aspects of caring for your batteries and how this applies to practical flying.
Most electric flyers wonder how long their model will fly with the battery they are using. They understand that if they fly too long, the low voltage cutoff (LVC) on their speed control may trigger and they will lose motor power and possibly control of their model. So we happily (nervously?) fly, land and wonder if we could have flown a little longer, or maybe we flew too long and are worried that we may have over discharging the pack. We can eliminate the “wondering” through the following simple technique.
I want to introduce you to the “Rule of 80”. This simply means that you should not discharge your battery beyond 80 percent of its capacity. Why? Because once the voltage on your pack falls to a certain level, it drops very quickly under load (propeller) and can damage the pack by over discharging it. This is a great way to “puff” your expensive battery and start it on its way to early retirement. The key here is the pack’s capacity. Neither the number of cells nor the “C” rating of your battery have anything to do with this principle. Say for example, you are flying a foamy warbird with a three cell 2600 milliamp capacity pack. Using the 80% rule, this would be 2600 X 0.8, which is 2080 milliamps. You should not take more than 2080 MaH out of the battery without the risk of over discharging.
Now, how do you put this to practical use? Easy! You only need a timing device. Most all newer transmitters have some sort of timing feature included. A simple digital kitchen timer works fine too, but in both cases you need to remember to start the timer when you takeoff and stop it after landing! Now, recharge the battery and note how many milliamps you put back in to fully recharge the pack. Using our 3S 2600 example from above, let’s say you put 1750 milliamps back in and flew a fairly aggressive (lots of loops, rolls and high speed passes) flight for six minutes. We simply divide the milliamps we put back into the pack by the pack’s capacity and multiply by 100 to get the percentage used.
So, 1750/2600 X 100 is equal to 67 percent of the pack’s capacity used.
That’s far enough below our 80% rule number that we know we can probably fly the model in the same manner again (it’s a warbird after all!) for the same length of time and probably a little longer with any battery of the same type. Repeat this process for each flight and in short order you will know the longest flight time you can expect from your pack. It’s important to keep track of your battery’s performance… all of them.
Final Note: the time you can fly is one area where the pack’s “C” rating may potentially become a factor (variable) as the amount of current that the motor and prop are demanding from the battery may exceed the pack’s design specs or if it’s an old, tired battery, it may not be up to the task. If your 2600 pack is rated at 20 “C”, it means that it can output a maximum of 44 amps (2.2 Ah X 20). A constant current draw at that rate can become an issue, even when the pack is fairly new. As our battery packs age it’s important to monitor them a little more closely. Always be on the lookout for a cell whose voltage is considerably higher or (more likely) lower than the others while charging. This is a sign that it may be going bad and extra care will need to be made when charging and discharging it. Eventually, all of our packs will need to be retired after hundreds of cycles but proper care and monitoring of your packs’ health can prolong that event for a many flying seasons.
At the next meeting, I am going to talk a little about chargers and the charging process.